Currently, standardized and commercially deployed radio access technologies are proliferated. Such radio access technologies include the Global System for Mobile communications (GSM), Enhanced Data Rates for GSM Evolution (EDGE), General Packet Radio System (GPRS), wide-band code division multiple access (WCDMA), Long Term Evolution (LTE) systems, wireless local area networks (WLAN), CDMA 2000 and others.
The radio channels in such wireless systems may vary or deteriorate over time and frequency. This is due to e.g. propagation gain, as transmitted radio signals reflects and produces reflected signals that may interfere with each other in a constructive or destructive way. Thus the signals may amplify or become impaired by the interference from reflected radio waves.
There are several known techniques to adapt to, and utilize this phenomenon. In particular, these techniques may include channel dependent scheduling and link adaptation.
Channel dependent scheduling can be used to schedule transmissions to and from a terminal at time and/or frequency instants for which the radio channel is particularly good.
Link adaptation adapts transmission parameters, typically modulation and channel coding, in order to reach some desired objective, e.g. to maximize bit rate or to minimize delay.
Both these techniques rely on measurements of the radio channel. Such measurements are done some time before they are used by the scheduling and link adaptation mechanisms. Since the radio channel varies over time, and because of the delay in time, the measurements will not be fully accurate when used. To mitigate this, a margin can be used, so that even if the radio channel deteriorates somewhat, the applied transmission parameters are appropriate. However, using such margin also means that e.g. a modulation and encoding scheme which is unnecessarily robust may be selected, instead of modulation and coding scheme allowing for a higher bit rate transfer.
In conventional wireless systems the scheduled bandwidth is constant. In such cases a fixed margin works satisfactory. In many modern wireless systems however, employing frequency domain scheduling, the allocated bandwidth differs dramatically. A consequence of this is that the channel variations between measurement and usage also differ. In such cases using a single margin may lead to performance loss as the margin has to be rather big to cover the variations over time of the radio channel.